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http://www.eurekalert.org/releases/emry-wdf100499.html
EMBARGOED FOR RELEASE: 6 OCTOBER 1999 AT 14:00 ET US
Contact: Sarah Goodwin
sgoodwi at emory.edu
404-727-5686
Emory University Health Sciences Center
We do 'feel with the mind's eye,' confirm Emory researchers in Nature
For the first time, researchers have verified that the part of the brain
involved in processing the sense of sight is also necessary for the sense of
touch. Results of an Emory University study confirming the role of visual
cortex in tactile (touch) perception are reported in this week's issue of
the journal Nature.
"The findings are relevant to understanding not only how the brain normally
processes sensory information, but also how such processing is altered in
conditions such as blindness, deafness or numbness and ultimately, to
improving methods of communication for individuals afflicted with these
disorders," says lead author Krishnankutty (Krish) Sathian, M.D., Ph.D.,
associate professor of neurology and joint associate professor of
rehabilitation medicine at the Emory University School of Medicine and a
faculty member in the interdisciplinary Neuroscience Program at Emory
University.
Until recently, scientists believed separate brain regions processed
information gathered by the various senses. This view is now being
challenged.
"The kind of interaction among the senses identified in our work may be more
common than generally appreciated," Dr. Sathian says. "Recent findings that
visual cortex is active during Braille reading in the blind are perhaps less
surprising if viewed in this context."
In one of the tasks used in Dr. Sathian's laboratory, a grooved object is
impressed onto the fingertip (grating) of human volunteers. With their eyes
closed or blindfolded, the subjects attempt to distinguish, via touch, the
orientation of the grooves, i.e., the direction in which the grooves run
(along vs. across the finger).
"People who performed this task told us they were visualizing 'with the
mind's eye' the orientation of the grating on the fingertip, suggesting to
us that visual imagery facilitates this tactile task," Dr. Sathian says. "I
had thought for some time that visual imagery might be involved in tactile
perception. We decided to see if we could obtain more direct evidence for
this."
The Emory group then used positron emission tomography (PET) to show that a
region of the cerebral cortex associated with sight is engaged when humans
attempt to distinguish orientation via delicate touch, as in the fingertip
grating task (NeuroReport, 1997). But whether this region was truly
necessary for tactile performance was unclear.
The current study aimed to answer this question. The researchers asked
normally sighted volunteers to perform a series of tactile discrimination
tasks designed to quantify tactile abilities, including the fingertip
grating task. Subjects were asked to tell either the orientation (direction)
of the grating (along or across the finger) or whether the grooves were wide
or narrow (spacing task). The 14 subjects performed the tasks while
researchers used harmless transcranial magnetic stimulation to transiently
block the function of, first, various parts of the brain region associated
with the sense of sight (the occipital cortex), and then, the brain region
mediating the sense of touch (the somatosensory cortex).
When a key region of visual cortex (the parieto-occipital cortex) was
blocked, subjects were significantly less successful in discriminating the
orientation of the grating via touch, according to first author of the paper
Andro Zangaladze, M.D., Ph.D., now a resident in neurology at Thomas
Jefferson University Hospital in Philadelphia, who conducted the research
while a fellow in Dr. Sathian's laboratory at Emory. However, performance on
the spacing task was unaffected, implying that the effect was selective for
orientation. In contrast, blocking somatosensory cortex interfered
nonselectively with performance on both tasks.
"Together with the subjective reports of visual imagery in this task and the
associated parieto-occipital cortical activation noted previously, our
findings support the proposal that visual processing facilitates normal
tactile discrimination of orientation," the authors write. "Perhaps this is
related to the fact that we generally rely on the visual system for
orientation discrimination. Thus, involvement of the visual cortex may be
beneficial when macrogeometric features such as orientation are to be
discriminated, but not for microgeometric features such as texture."
Co-authors of the study include Charles M. Epstein, M.D., and Scott T.
Grafton, M.D., both associate professors in Emory's neurology department.
"This study exemplifies the kind of collaboration that is so characteristic
of Emory," Dr. Sathian says. "Apart from my own background in sensory
physiology and psychophysics, the work drew on the expertise of Dr.
Zangaladze in human electrophysiology, of Dr. Epstein in transcranial
magnetic stimulation and of Dr. Grafton in functional brain imaging."
The work was supported by research grants to Dr. Sathian from the National
Institute of Neurological Disorders and Stroke and the National Eye
Institute of the National Institutes of Health.
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